Several types of charged-particle-beam (CPB) optical systems have been developed for high-resolution microlithography. These include the symmetrical magnetic doublet, described in, for example, M. B. Heritage, "Electron-Projection Microfabrication System," J. Vac. Sci. Technol. 12, pp. 1135-1140 (1975), the moving objective lens ("MOL") system, described in, for example, H. Ohiwa, "Design of Electron-Beam Scanning System Using the Moving Objective Lens," J. Vac. Sci. Technol. 15, pp. 849-85 (1978). Other CPB optical systems include the variable axis lens ("VAL") system discussed in, for example, H. C. Pfeiffer et al., "Variable Axis Lens for Electron Beams," Appl. Phys. Lett. 39, pp. 775-776 (1981) and the projection exposure with variable axis immersion lenses ("PREVAIL") system as described in, for example, H. C. Pfeiffer, "Projection Exposure with Variable Axis Immersion Lenses: A High-Throughput Electron Beam Approach to `Suboptical` Lithography," Japan. J. Appl. Phys. (Part 1) 34, pp. 6685-6662 (1995) (hereinafter "Pfeiffer").
Because of aberrations in CPB optical systems, the entire pattern required for a complex integrated circuit such as a memory chip cannot be projected from a mask to a wafer in a single exposure. Therefore, the mask containing the circuit pattern is divided into fields containing circuit patterns for a single chip or die. The fields are further divided into subfields such that the CPB optical system aberrations are acceptable for projection of these subfields. Images of the subfields are then individually and sequentially projected onto the sensitized wafer and joined together so that an image of the complete pattern is formed. The images of the subfields are joined by combining mechanical scanning of the mask and/or wafer and deflection of the image formed by the charged-particle beam.
With a symmetric magnetic doublet, aberrations on the optical axis and over a large field of view tend to be small. However, field curvature and astigmatism are large and can seriously degrade the images formed, especially for large lens apertures. If the lens apertures are small, then the interactions of the charged particles with each other (e.g., electron-electron interactions) in the charged-particle beam degrade the images. If the current density is reduced in order to avoid electron-electron interactions, then throughput decreases. As a result, the symmetric magnetic doublet is unable to simultaneously provide high resolution and high throughput.
In the VAL, PREVAIL, and MOL systems, the location of the optical axis is effectively shifted so that the charged-particle beam propagates along an effective optical axis even for off-axis points. Large apertures and small aberrations can be achieved and throughput is high. For example, in the PREVAIL system of Pfeiffer, deflectors are provided that produce supplemental magnetic fields so that the total magnetic field encountered by electrons propagating off-axis is approximately the same as the magnetic field near the optical axis. In this way, the field of view of the CPB optical system is increased. The supplemental magnetic fields satisfy conditions so as to create an effective optical axis that is displaced from the optical axis. As a result, off-axis field points are imaged with aberrations of the same magnitude as an on-axis point.
In these systems, a single deflector directs the charged-particle beam through a crossover aperture on the optical axis of a first lens and a single deflector redirects the charged-particle beam received from the crossover so that the beam propagates parallel to the optical axis of a second lens. These deflectors must produce large deflections. If the effective optical axis is displaced a large distance from the optical axis, deflection aberrations become significant.
Pfeiffer shows an exemplary PREVAIL system in which axis-shifting deflectors create an effective optical axis displaced from the optical system. The electron beam propagates along the effective optical axis of a condenser lens that directs the electron beam to the mask; projection lenses image the mask onto the wafer. Subfield-selection deflectors direct the electron beam so that a selected mask subfield is irradiated and direct the image of the mask subfield onto a corresponding transfer subfield of the wafer.
The PREVAIL system has several significant limitations. In the PREVAIL system, imaging errors caused by electron-electron interactions are reduced to some extent by reducing the distance between the mask and the wafer. Generally, however, the mask-wafer distance is still too large and charged-particle interactions limit image resolution. In addition, if the distance between the positioning deflectors and the aperture stop is small, then large deflections are required, resulting in large deflection aberrations. If the distances are reduced to control deflection aberrations, then the axis-shifted lens must be shorter, thereby increasing on-axis lens aberrations. The subfield-selection deflectors must be precisely controlled and a well regulated power supply is necessary to drive these deflectors. Because the power supply must be precisely controlled, these deflectors cannot be used for rapid repositioning of the electron beam.
Although the aberrations of the lenses used in the MOL, VAL, VAIL, and PREVAIL systems are improved, aberrations caused by manufacturing errors of the deflectors are not eliminated and these systems exhibit large distortions. As a result, none of these systems are suited for the production high-density integrated circuits such as dynamic random-access memory (DRAM) of 4 Gbit or more.
Therefore, apparatus and methods are needed for transferring patterns from a mask to a sensitized substrate with increased resolution and high throughput. In particular, a CPB optical system is needed that can project a large subfield with low levels of aberrations and that has reduced sensitivity to manufacturing errors.